A pyrometer may be used to measure temperature of an object without making physical contact with the object. For instance, radiation from an object may be focused onto a detector, whose output is related to irradiance of the object. The temperature of the object may be inferred from the measured irradiance and known emissivity of the object.
A multi-spectral pyrometer eliminates the need to know the emissivity of an object. Temperature measurements by a multi-spectral pyrometer are based on a gray body assumption whereby emissivity at different wavelengths is assumed to be constant. However, the gray body assumption is not valid for metals and other materials that do not have the same emissivity at two different wavelengths, thus leading to erroneous results if used to measure a multi-temperature profile.
Consider the example of low emissivity glass manufacturing, where different grades of low emissivity glass have very low, yet different, emissivity values. These emissivity values may not be known, especially at their respective processing temperatures. A multi-spectral pyrometer would produce an erroneous profile.